3GPP LTE adopts OFDMA (Orthogonal Frequency Division Multiple Access) as a downlink communication scheme. In a radio communication system to which 3GPP LTE is applied, a base station apparatus (hereinafter, abbreviated as “base station”) transmits a synchronization signal (i.e., Synchronization Channel: SCH) and broadcast signal (i.e., Broadcast Channel: BCH) using predetermined communication resources. A terminal apparatus (hereinafter abbreviated as “terminal”) locates an SCH to secure synchronization with the base station first. After that, the terminal reads BCH information to acquire base station-specific parameters (e.g., frequency bandwidth) (see Non-Patent Literatures 1, 2, and 3).
Upon completion of the acquisition of the base station-specific parameters, the terminal sends a connection request to the base station to thereby establish communication with the base station. The base station transmits control information to the terminal with which communication has been established via a PDCCH (Physical Downlink Control CHannel) as required.
The terminal then makes a “blind decision” on a plurality of control information portions included in the received PDCCH signal. That is, each of the control information portions includes a CRC (Cyclic Redundancy Check) portion and the base station masks this CRC portion with a terminal ID of the transmission target terminal. Therefore, the terminal cannot make a decision on whether the received control information portion is addressed to the terminal or not until the terminal demasks the CRC portion of the received control information portion with the terminal ID of the terminal. In the blind decision, if the demasking result shows that the CRC operation is OK, the control information portion is judged as being addressed to the terminal apparatus.
Furthermore, in 3GPP LTE, ARQ (Automatic Repeat Request) is applied to downlink data from the base station to the terminal. That is, the terminal feeds back a response signal indicative of an error detection result of the downlink data to the base station. The terminal performs a CRC on the downlink data and feeds back ACK (Acknowledgment) when CRC=OK (no error) or NACK (Negative Acknowledgment) when CRC=NO (error) to the base station as a response signal. Here, BPSK (Binary Phase Shift Keying) is used to modulate the response signal (that is, ACK/NACK signal). Furthermore, an uplink control channel such as PUCCH (Physical Uplink Control Channel) is used to feed back the response signal. When the received response signal indicates NACK, the base station transmits retransmission data to the terminal.
Here, the control information transmitted from the base station (that is, downlink allocation control information) contains resource allocation information including resource information or the like allocated to the terminal by the base station. The above-described PDCCH is used to transmit the control information. The PDCCH is formed of one or a plurality of L1/L2 CCHs (L1/L2 Control Channels). Each L1/L2 CCH is formed of one or a plurality of CCEs (Control Channel Elements). That is, the CCE is a base unit when control information is mapped to the PDCCH. Furthermore, when one L1/L2 CCH is formed of a plurality of CCEs, the plurality of CCEs having serial identification numbers (indices) are assigned to the L1/L2 CCH. The base station allocates the L1/L2 CCH to a resource allocation target terminal according to the number of CCEs necessary to notify the resource allocation target terminal of control information. The base station then transmits control information mapped to physical resources corresponding to CCEs of the L1/L2 CCH.
Here, CCEs are associated with component resources of the PUCCH in a one-to-one correspondence. Therefore, the terminal that has received the L1/L2 CCH can implicitly identify the component resources of the PUCCH corresponding to the CCEs constituting the L1/L2 CCH and transmits a response signal to the base station using the identified resources. However, when the L1/L2 CCH occupies a plurality of consecutive CCEs, the terminal transmits a response signal to the base station using one of the plurality of PUCCH component resources corresponding to the plurality of CCEs (e.g., PUCCH component resource corresponding to a CCE having the smallest index). Downlink communication resources are thereby used efficiently.
A plurality of response signals and reference signals transmitted from a plurality of terminals are spread on a time axis (i.e., time domain) using a ZAC (Zero Auto-correlation) sequence (may also be called “base sequence”) having a Zero Auto-correlation characteristic and Walsh code sequence or DFT (Discrete Fourier Transform) sequence as shown in FIG. 1, and code-multiplexed within a PUCCH (however, a ZAC sequence having a sequence length of 12 itself may also be called “reference sequence”).
In FIG. 1, (W0, W1, W2, W3) represents a Walsh sequence (Walsh code sequence) having a sequence length of 4 and (F0, F1, F2) represents a DFT sequence having a sequence length of 3. As shown in FIG. 1, in the terminal, an ACK or NACK response signal is primary-spread within 1 SC-FDMA symbol on the frequency axis by a ZAC sequence (having a sequence length of 12, may also be referred to as “Base Sequence”) first. Next, the primary-spread response signals are associated with W0 to W3 respectively and subjected to IFFT (Inverse Fast Fourier Transform). Furthermore, in the terminal, a ZAC sequence having a sequence length of 12 and serving as a reference signal is associated with F0 to F2 respectively and subjected to IFFT. Thus, the response signal and reference signal spread using the ZAC sequence having a sequence length of 12 on the frequency axis (Frequency domain) and the reference signal are converted to a ZAC sequence having a sequence length of 12 on the time axis through IFFT. This is equivalent to a primary-spread response signal and the reference signal after IFFT further being secondary-spread using a Walsh sequence (sequence length of 4) and a DFT sequence (sequence length of 3).
Response signals from different terminals are spread using ZAC sequences corresponding to different amounts of cyclic shift (cyclic shift indices) or orthogonal code sequences corresponding to different sequence numbers (orthogonal cover indices: OC indices). An orthogonal code sequence is a combination of a Walsh sequence and a DFT sequence. Furthermore, the orthogonal code sequence may also be referred to as “block-wise spreading code sequence.” Therefore, the base station can demultiplex a plurality of code-multiplexed response signals, using conventional despreading and correlation processing (see Non-Patent Literature 4).
However, since each terminal makes a blind decision on downlink allocation control information addressed to the terminal apparatus in each subframe (transmission unit time), reception of downlink allocation control information is not always successful on the terminal side. When the terminal fails to receive downlink allocation control information addressed to the terminal in a certain downlink component carrier, the terminal cannot even know whether or not downlink data addressed to the terminal exists in the downlink component carrier. Therefore, when the terminal fails to receive downlink allocation control information in a certain downlink component carrier, the terminal does not generate any response signal for downlink data in the downlink component carrier either. This erroneous case is defined as DTX (Discontinuous transmission of ACK/NACK signals) of response signals in the sense that the terminal does not transmit any response signal.
Furthermore, standardization of 3GPP LTE-advanced for realizing faster communication speed than 3GPP LTE has started. 3GPP LTE-advanced systems (hereinafter may also be referred to as “LTE-A systems”) follow 3GPP LTE systems (hereinafter may also be referred to as “LTE systems”). 3GPP LTE-advanced is expected to introduce base stations and terminals communicable at a wideband frequency of 40 MHz or more to realize a downlink transmission rate of a maximum of 1 Gbps or above.
To simultaneously realize an ultra-high-speed communication several times faster than transmission rates in LTE systems and backward compatibility with LTE systems, in LTE-A systems, LTE-A system bands are divided into “component carriers” of 20 MHz or below which is the support bandwidth of LTE systems. That is, the “component carrier” is a band having a width of a maximum of 20 MH and is defined as a base unit of communication band. Furthermore, a “component carrier” in a downlink (hereinafter referred to as “downlink component carrier”) may be defined as a band divided by downlink frequency band information in a BCH broadcast from the base station or a band defined by a distribution width in the case where a downlink control channel (PDCCH) is distributed in a frequency domain. Furthermore, a “component carrier” in an uplink (hereinafter referred to as “uplink component carrier”) may also be defined as a band divided by uplink frequency band information in a BCH broadcast from the base station or a base unit for a communication band of 20 MHz or below including a PUSCH (Physical Uplink Shared CHannel) region near its center and PUCCHs for LTE at both ends. Furthermore, the “component carrier” in 3GPP LTE-Advanced may be expressed in English as Component Carrier(s), and may also be defined by a physical cell number and carrier frequency number, and may be called “cell.”
The LTE-A system supports communication using a band that bundles several component carriers, so-called “carrier aggregation.” Throughput requirements for an uplink are generally different from throughput requirements for a downlink. In the LTE-A system, carrier aggregation in which the number of component carriers set for any terminal supporting an LTE-A system (hereinafter referred to as “LTE-A terminal”) is different between the uplink and the downlink, so-called “asymmetric carrier aggregation” is also being discussed. Furthermore, the LTE-A system also supports configurations where the numbers of component carriers are asymmetric between the uplink and downlink, and the component carriers have different frequency bandwidths.
FIG. 2 is a diagram illustrating asymmetric carrier aggregation and a control sequence applied to individual terminals. FIG. 2 shows an example where bandwidths and the numbers of component carriers are symmetric between an uplink and a downlink of a base station.
In FIG. 2B, a setting (hereinafter, referred to as a configuration) is made for terminal 1 such that carrier aggregation is performed using two downlink component carriers and one uplink component carrier on the left side, whereas a configuration is made for terminal 2 such that although the two same downlink component carriers as those in terminal 1 are used, one uplink component carrier on the right side is used for uplink communication.
Referring to terminal 1, an LTE-A base station and an LTE-A terminal included in an LTE-A system transmit and receive signals to and from each other according to a sequence diagram illustrated in FIG. 2A. As illustrated in FIG. 2A, (1) terminal 1 is synchronized with the downlink component carrier on the left side when starting communication with the base station, and reads information on the uplink component carrier from a broadcast signal called “SIB2 (system information block type 2),” the uplink component carrier forming a pair with the downlink component carrier on the left side. (2) Using this uplink component carrier, terminal 1 starts communication with the base station by transmitting, for example, a connection request to the base station. (3) Upon determining that a plurality of downlink component carriers need to be assigned to the terminal, the base station instructs the terminal to add the downlink component carriers. However, in this case, the number of uplink component carriers does not increase, and terminal 1 which is an individual terminal starts asymmetric carrier aggregation.
Furthermore, in the above LTE-A system to which carrier aggregation is applied, the terminal may receive a plurality of downlink data portions in a plurality of downlink component carriers at a time. In LTE-A systems, studies are being carried out on a technique of reducing a spreading factor of response signals in a PUCCH (so-called SF (Spreading Factor) reduction) as one of a plurality of response signal transmission methods for the plurality of downlink data portions.
To be more specific, as shown in FIG. 3, two response signals (Symbol 1 and Symbol 2) are generated on the terminal side, the two response signals are spread by Walsh code sequences (W0,0, W0,1) and (W1,0, W1,1) having a sequence length of 2 respectively and arranged on first and second SC-FDMA symbols, and sixth and seventh SC-FDMA symbols respectively. By so doing, even when the terminal receives a plurality of downlink data portions in a plurality of downlink component carriers at a time, the terminal can feed back a plurality of response signals for the plurality of downlink data portions to the base station at a time.